Step 1: 1. Preheat the oven to 200c
Step 2: 2. Melt the butter in a pan and add the onion and cook for about 5 minutes or until the onion is translucent.
Step 3: 3. Stir in the flour and add the milk a small amount at the time and stir constantly to get a smooth sauce. Once you have got a smooth thick sauce remove it from the heat
Step 4: 4. Add the fish and lemon juice to the sauce and season
Step 5: 5. Cook and wash the spinach by covering it with boiling water and leaving it to cook for a few minutes before draining thoroughly
Step 6: 6. Grease the bottom of a oven proof baking dish and add a 1/3 of the fish sauce, then add a layer of spinach and the lasagna sheets.
Step 7: 7. Repeat until you have used up all the layers and finish with the fish sauce on the top.
Step 8: 8. Sprinkle with parmesan and bake in the oven for 45 minutes.